1. Field of the Invention
The present invention relates to an image display apparatus.
2. Related Background Art
FIG. 1 is a schematic view showing the principal structure of an example of the conventional image display apparatus.
Said display apparatus is provided with a light source 1 composed for example of a halogen lamp or a metal halide lamp; a mirror 2 reflecting a part of the light emitted by said light source 1; a heat ray cut-off filter 3 for absorbing or reflecting the heat ray in the light entering directly from the light source 1 or indirectly from the mirror 2; a condenser lens 4 for converting the light, after removal of the heat ray, into a parallel beam; a polarizer 5 for converting said parallel light beam into linearly polarized light; a liquid crystal light valve 7 for modulating said linearly polarized light according to an image signal; a polarizer 8 for transmitting only a component, parallel to the transmission axis thereof, of said modulated linearly polarized light; and a projection lens 10 for projecting the linearly polarized light, transmitted by said polarizer 8, in a magnified scale onto an unrepresented screen.
FIG. 2 is a schematic view showing the principal part of another example of such conventional projection display apparatus.
Said apparatus is equipped with two polarizing beam splitters 6, 9 respectively in front of and behind the light crystal light valve 7, in place for the two polarizers 5, 8 in the apparatus shown in FIG. 1.
The projection display apparatus shown in FIGS. 1 and 2 are associated with a drawback that the efficiency of ulitization of light does not exceed 50%, since, within the light emitted by the light source 1, a linearly polarized component transmitted by the polarizing beam splitter 6 alone is utilized for illuminating the light crystal light valve 7 while the perpendicularly polarized component is lost.
FIG. 3 shows a projection display apparatus disclosed in the Japanese Patent Application Laid-Open No. 61-90584 for rectifying said drawback.
In said projection display apparatus the parallel light beam emerging from the condenser lens 4 enters a polarizing beam splitter 11, and the P-polarized component L.sub.p is transmitted by the functional plane (an evaporated film formed on a diagonal plane between two rectangular prisms) 11a of said polarizing beam splitter 11, while the S-polarized component L.sub.s is perpendicularly reflected to enter a total reflection prism 12. Being perpendicularly reflected again in said prism 12, the S-polarized component L.sub.s emerges from said prism 12 in a direction same as that of the P-polarized component L.sub.p. The S-polarized component L.sub.s is polarized in a direction parallel to the functional plane 11a of the polarizing beam splitter 11, and the P-polarized component L.sub.p is polarized in a direction perpendicular to that of the S-polarized component.
At the exit side of the total reflection prism 12 there is provided a .lambda./2-phase shifting plate 13, whereby said S-polarized component L.sub.2 is subjected to a rotation of the polarizing direction by 90.degree. and is converted into a P-polarized component L.sub.p *. Also at the exit side of the polarizing beam splitter 11 and the .lambda./2-phase shifting plate 13 there are respectively provided wedge-shaped lenses 14, 15 for light path deflection, whereby the P-polarized component L.sub.p transmitted by said polarizing beam splitter 11 and the P-polarized component L.sub.p * converted by the .lambda./2-phase shifting plate 13 are subjected to light path deflection and mutually cross at a point P.sub.0 on the entrance face of the liquid crystal light valve 7, thereby providing a synthesized light.
Consequently such projection display apparatus can illuminate the liquid crystal light valve 7 with both the S-polarized component L.sub.s and the P-polarized component L.sub.p separated by the polarizing beam splitter 11 and can therefore double the efficiency of light utilization in comparison with the apparatus shown in FIG. 2.
However, in the projection display apparatus disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 61-90584, since the P-polarized component L.sub.p and the P-polarized component L.sub.p * converted by the .lambda./2-phase shifting plate 13 respectively enter the liquid crystal light valve with an angle .theta. as shown in FIG. 3, it is necessary to select a considerably large distance from the wedge-shaped lenses 14, 15 to the liquid crystal light valve 7 in order to reduce said incident angle .theta. if the light valve 7 shows significant deterioration of characteristics depending on the incident angle.
For avoiding such drawback, there is conceived a parallel illuminating method in which the wedge-shaped lenses 14, 15 shown in FIG. 3 are removed, whereby said P-polarized component L.sub.p and said converted P-polarized component L.sub.p * enter the liquid crystal light valve 7 in mutually parallel state. However, such parallel illumination method, if applied to the projection display apparatus disclosed in the Japanese Patent Application Laid-Open No. 61-90584, cannot provide the expected result because the P-polarized component L.sub.p and the converted P-polarized component L.sub.p * are not complete unless the light source 1 is a complete point or linear source providing completely parallel beams from the condenser lens 4. This will be explained further with reference to FIG. 4.
In case the light from a light source 1 with a finite diameter .phi. is condensed by a condenser lens 4 at a distance l, the light emerging therefrom is not completely parallel but is spread within an angular range 2.omega.(.omega.=tan.sup.-1 ((.phi./2)/l). A ray .alpha. contained in thus obtain non-parallel beam enters the .lambda./2-phase shifting plate 13 without the function of the polarizing beam splitter 11 and emerges from said phase shifting plate 13 with the P- and S-polarized components. Also a ray .beta. is converted by the polarizing beam splitter 11 into the S-polarized component L.sub.s, which is then reflected by the total reflection prism 12 and reflected again by the polarizing beam splitter 11. It thus emerges as a P-polarized component L.sub.p * from another position of the .lambda./2-phase shifting plate 13 as indicated by a ray .beta..sub.1, or is lost by absorption or transmission at the surface of the phase shifting plate 13 as indicated by a ray .beta..sub.2.